1985 Yamaha 90 hp Water pump Question

[Timmstng67]

Hi
I just rebuilt my water pump with a new rebuild kit. I reinstalled lower unit and tested in my tank it would not pump water. If I put the flush ear muffs on long enough for it to prim the system it will pump water on its own no problem. When I shut it down you can hear the cooling system draining down. When restarted it will not pick up water again until it is primed. Stumped?
Any help would be appreciated.

Thanks Tim